As explained in the cross-referenced patent application bilateral switches have been used in the unrestricted frequency changer (UFC) to sequentially connect input and output AC lines for both directions of flow. While so doing with a proper relation between opposite phase lines through appropriate such bilateral switches, it is possible to treat switch modulation like on an inverter working with a "hidden" DC link input, thereby to create an AC/AC converter working on the principle of the conventional double conversion system i.e. AC/DC and DC/AC converter interconnect by a DC link.
The present invention provides for current and frequency control in a system embodying AC/AC conversion according to the copending case. In its preferred embodiment, use is made of the bang-bang technique. The bang-bang technique is well known. See for instance U.S. Pat. Nos. 3,636,430 and 3,648,150 which relate to inverter control and involve pulse-width modulation.
The invention is particularly applicable to AC motor drives, and it will be described in this context.